Interpolation/differentiation on parallelepiped grids

[PaulWAyers]

Currently our interpolation for "cubic" grids works only when the coordinate axes are orthogonal. We should extend this to non-orthogonal cell vectors.

@Ali-Tehrani I know you had been working on this, so let me know if you, or I, should flesh this issue out with details on how this is achieved.

[PaulWAyers]

This can be done as a post-processing of #261

[dhruvDev23]

Hi @PaulWAyers , I'd like to try. Could you please assign this issue to me?

[PaulWAyers]

Sure, take a crack at it. I thought about it once before, but I'm not sure I still have my notes. The basic idea was to work out the Jacobian matrix of the transformation relative to a cubic grid, as I recall.